Variable resistor



C. F. SCHAEFER July 17, 1951 VARIABLE REsIsToR Filed July 30, 1949 2 Sheets-Sheet l C am F SCHHEFE@ FITTOE/EY JNVENToR. C me?. f1' Swan-Ee ATTORNEY 2 Sheets-Shea?I 2 w H n, /l O /Alz a C F SCHAEFER VARIABLE RESISTOR July 17, 1951 Filed July 30, 1949 Patented July 17, 1951 UNITED STATES PATENT OFFICE VARIABLE RESISTOR Carl F. Schaefer, Port Washington, N. Y., assignor to The Norden Laboratories Corporation, New York, N. Y., a corporation of Connecticut.

Application July 30, 1949, Serial No. 107,659

13 Clams..y l

My invention relates to variable resistors and more particularly to a variable resistor enabling me to adjust a resistance with improved accuracy. Most variable resistors comprise a re sistance element such as a winding of a resistance wire of a nickel chrome alloy or the like upon an insulating base or core together with a brush movable over the resistance element. The length of the resistance element between a terminal and the brush determines the particular resistance for the setting. It has been very dife cult in practice to produce a variable resistor having linearity throughout and a desired overall resistance. It is possible to adjust xed re.- Sstors to give the desired overall resistance within very narrow limits. The resistance element, however, varies from point to point so that the resistance will not necessarily vary as a linear.

function of the movement of the brush over the resistance element. In many applicationssuch as phase shifters and electronic computing instruments it is extremely desirable to have a variable resistor which will have not only the desired overall resistance but in which the resistance can be varied with greater accuracyto produce a linear function.

One object of my invention is to provide a variable resistor having the desired overall resistance within narrow limits and which will have greater improved linearity, so that a pre.- determined movement of the variable brush will produce a corresponding change in resistance. within narrow limits.

Other and further objects of my invention will appear from the following description:

In general, my invention contemplates the division of the desired overall resistance of a variable resistor into a plurality of increments of fixed resistors, each of which may be adjusted to have a desired resistance within close limits. A variable resistor having overall resistance equal to one-half of the increments is provided with a pair of brushes spaced 180 apart'. The brushes are moved in synchronism with a Switch apparatus adapted to switch the xed resistors cumulatively in series with the rheostat element in timed relation, so that I need only to accept non-linearity of a small fraction of the overall resistance and thus enable me to achieve a variable resistor having a resistance variable over a wide range with a high degree of accuracy.

In the accompanying drawings which. form part of the instant specification and which are to be read in conjunction therewith, and in which like reference numerals are used to indicate like parts in the various views:

Figure l is a diagrammatic view showing a resistor containing one embodiment of my invcntion.

Figure 2 is a.' perspective schematic View showz ing the construction of the resistor shown in Figure 1,.

Figure 3 is a graph of resistance plotted against degrees of rotation of the variable brush showing how an adjustment in the resistance of the resistor with which the brush cooperates is made.

Referring now tol the drawings, a pair of ter'-4 minals 4 and 6 are provided across which the resistance is measured. A conductor 8 connects the terminal 6 to an arcuate resistor I0 of a conventional rheostat construction which may be formed of any suitable resistance element wound to give the best linearity that can be achieved with a resistor of this construction. The arc of the resistance element l0 may be about 200. A central shaft l2 supports an insulating arm I4 upon which are mounted a pair of brushes I6 and I8 which are positioned 180 apart. The resistance element Ill is mounted on a suitable insulating base 20 which is provided with a slot 22 permitting displacement of the resistance winding I0 with respect to the brushes, so that the brushes may include more or less of the resistance element between their points of contact for purposes of adjustment, as will be pointed out hereinafter more fully. The value of the resistance included between the brushes I8 and I8 is a predetermined value, say R. The

brush I6 is connected by a conductor 24 to a plurality of resistance elements ,25, 28, 3D, 32, 34, 36 and the like, which are connected in series. The value of the resistance of resistance element 26 is R. That is exactly the same as the value of the resistance of the resistor II) which is included between the brushes I6 and I8t The value of the subsequent resistors 28, 30, 32 and 34 are carefully calibrated to be 2R. The end of resistor 26 is connected by conductor 33 to a switch sector 40. The end of resistors 25 and 28 in series is connected by a conductor 42 to a switch sector 44. The end of resistors 26, 28 and 30 in series is connected by a conductor 46 to a switch sector 48. The end of resistors 26, 28, 38,' and 32 in series is connected by conductor 50 to a switch sector 52. Similarly the end of resistor 34 which is in series with resistors 32, 30, 28, and 28 is connected by conductor 54 to a switch sector 56. A symmetrically disposed group of switch sectors 58, 60, 62, 64, and 66 are provided to correspond with switch sectors 5t, 52, 48, 4 4, and 4I). Brush I8 is connected by a conductor 68 to the end of resistor 'l0 which is also connected by a conductor 'I2 to the switch sector 66. rI'he other-end of resistor 1), the value of the resistance of which is 2R, is connected by conductor 14 to the switch sector 64. Another resistor 'I6 of the value 2R is connected in series with resistor 'l0 and its end is connected by conductor 'I8 to the switch sector B2'.

Another resistor 80 having a resistance value of 2R is connected in series with resistors I0 and 16 and its end is connected by conductor 82 to the switch sector 60. A further resistor 64 connected in series with the resistors 80, 16, and 10, and its end is connected by a conductor 66 to the switch sector 58. There may be as many additional resistors, such as resistor 68, sim?.- larly connected to additional sectors as desired.. There are a pair of brushes 90 and 92 spaced 180 apart so that when brush 90 just makes contact with sector 40 moving in a clockwise direction, brush 92 will be at the lower portion of sector 66. Brush 92 will make contact with sector 64 before contact of brush 90 is broken with sector 40. The sectors are mounted upon a base member 94. In order to avoid a critical adjustment of the zero point an additional sector 39 is connected by conductor 69 to the brush 92. In this way, if the zero point varies from point 9 or if the microswitch |08 lags in its ac tion zero resistance will be registered either by brush I6 or by brush I8. The brushes 90 and 92 are carried by insulating arm 9| adapted to be rotated by a shaft 96. The shaft 96 is connected to a gear 98 which meshes with a gear |00. The gear meshes with a gear |02 which is secured to a shaft I2. The arrangement is such that when the brush I6 rotates 180 the reduction gearing will rotate the brush 92 to make contact with sector 64 and upon rotation of an additional 180 the brush 90 will make contact with sector 44. In other words, a 360 rotation of the arm I4 will move the arm 9| which carries brushes 90 and 92 from a point upon a sector 40 to a corresponding point upon adjacent sector 44. Similarly a rotation of 360 of the arm I4 will move the brush 92 from a point on sector E6 to a corresponding point upon sector 64. I'he sectors 56 and 40 are out of phase an equivalent of 180 of movement of arm I4.

The brush 92 is connected by conductor |04 to contact point |06 of a snap switch indicated generally by the reference numeral |08. The brush 90 is connected by a conductor I I0 to contact point I I2 of the snap switch |08. The mov able switch member |I4 which is adapted to make alternate contact with contact points |06 and II2 is connected by conductor I|6 to the variable resistor terminal 4. A shaft II8 is connected for rotation with the arm I4 of the rheostat element of the resistor. A cam is secured to and carried by shaft IIB for rotation therewith. The cam is shaped to coact with a follower |22 adapted to actuate the switch II4, such that every 180 of rotation of the arm I4 will throw the switch alternately to make contact with the other of the contact points. For example, in the position shown the switch arm ||4 has just been thrown to make contact with contact point |I2. In operation, let us assume that the parts are in position for a zero resistance. This will occur when brush I8 is at the point 9. In this position the switch arm |I4 should be in contact with the contact point |06. In practice an exact relationship between the position of brushes I6 and I8 and the point of operation of switch |08 is difficult to control. The switch arm I I4 may, therefore, be in contact with contact point II2, but zero resistance will still be recorded due to the interconnection of brush 92 and segment 39 by the conductor 89. The arrangement is such that the correct resistance will be included between terminals 6 and 4 as the switch operates at any time that 'LTI both brushes I6 and I8 are in contact with the resistance winding between point 9 and the end of the winding. For example, if there were 200 of winding, the nominal operating point might be set when one of the brushes was 10 from point 9 with a tolerance of plus or minus 10. The conductors will have a small but fixed resistance so that this constant is always additive to the resistance of my variable resistor. With the parts in the position described, the resistance across posts 4 and 6 is measured as follows: From post 6 through conductor 8 through brush I8 through conductor 68 through conductor 'I2 to sector 66 through brush 92 through conductor |04 to contact point |06 through switch arm II4 to the post 4. When brush I8 is at point 9 on the resistance winding I0, brush I6 will be 180 therefrom or at point II. The path from post 6 through conductor 8 through the resistor |0 to the brush I6 through the conductor 24 through the resistor 26 through the conductor 38 to the sector 40 and thence to brush 90 and conductor IIO to contact post II2 is open since the synchronization of the parts is such that when brush I8 is at point 9 brush I6 will be at point II and the switch arm II4 Will be in contact with post |06, as just pointed out. Let us now trace the operation of my resistor. As the arm I4 of the rheostat element thereof is rotated in a clockwise direction, as indicated by the arrow in Figure 1, the resistance across the posts 4 and 6 will gradually increase until it reaches a predetermined value of R which occurs when the brush I6 reaches a point, say point II, on the resistance winding I0. When this occurs the brush I6 will have reached the point 9 and the cam |02 will operate the switch arm II4 so that it will make contact with contact post II2. The resistance across the posts 4 and 6 will then be measured from post 6 through conductor 8 through brush I6 through conductor 24 through the resistor 26, the value of which will be observed is R, exactly equal to the arcuate segment of the resistance winding I0 traversed by the r brush I8 up until the instant the cam was operated. Continuing the path through which the resistance is measured, we have the conductor 38, the sector 40, the brush 90, the conductor IIO, the contact post |I2, the switch arm I|4,

A the conductor ||6 and iinally post 4. It will be observed that in this position the other resistance path from post 6 through the rheostat resistor I0, brush I6, conductor 68, conductor 12, sector 66, brush 92, conductor |04 to post |06 is open` As the rotation continues a resistance proportional to the rotation will be included in the resistance path in series with the resistor 26; When the brush I6 reaches the point II this re sistance will be exactly 2R and at this instant the brush I8 will be in contact with the point 9 and the cam will operate the switch arm I I4, so that it will contact contact point |06. The ro-4 tation of the arm I4 is transmitted by shaft I2 to the gear |02 which will idle the gear |00 in` a counterclockwise direction and the gear 98 in a clockwise direction. The gear ratios are such that just prior to the throwing of the switch arm I I4 to bring it into contact with contact point |06 the brush 92 will have moved to make contact with sector 64. The resistance path then will be from terminal post 6 through conductor 8 through brush I8 through conductor 68 through resistor 'I0 through conductor 'I4 through sector 64 through brush 92, throughconductor |04 to contact point |06 through switch arm II4,-con

ductor i I6 and post l. It will be observed that the resistor is 2R which is the precise equivaient of the resistor 26 plus the added resistance introduced by the traversal of the brush I6 over the arcuate resistor I0. Continued rotation will increase the resistance in increments of R proportional to the rotation of the arm I4. Every 180 at a point when the resistance traversed equals R a switching will occur substituting at that instant a fixed resistance whose value increases the former fixed resistance by the value o1 the resistance just added. The process may continue to any desired resistance as expressed in multiples of R. In the resistor illustrated in Figure 1, 10R is the maximum resistance which can be obtained with the sectors shown. Additional sectors and additional resistors similar to 38 and 88 may be added as desired. It will be clear that when the arm I4 is turned in a counterclockwise direction the resistance is decreased in a manner similar to that described above, and fixed resistors are cut out of the resistance path in multiples of R for every 180 of counterclockwise rotation of the arm I4 of the rheostat element of my resistor.

It will be readily appreciated by those skilled in the art that it is extremely important that the overall resistance of the arcuate resistance element II) which is swept by a brush to the point at which the switch takes place, that is, the value `oi the resistance embraced between brushes I5 and I0 must be exactly equal to R. Since it is extremely diiicult to make a resistance element exactly linear, and since the brushes must be maintained at a position 180 from each other in order not to disturb the phase relationship in the switching, there must be means for calibrating the value of the resistance embraced. between the brushes to equal the value R. I do this in a simple and convenient manner.

Referring now to Figure 2, it will be observed that the base member 2li is provided with a slot 22 so that it may be adjusted to give a relative motion between the arm I4 and the geometric center of the circle along the locus of which the resistance l0 is positioned. The resistor I0 furthermore is wound so that it has a denite width of an appreciable area to the end that if the base is moved so that its center is eccentric from the axis of rotation of the shaft I2 which lcarries the arm I4, the brushes I6 land I8 will still make contact with the resistance wire. If the base member 20 is moved downwardly the `arm III and the brushes I6 and I8 will occupy the position indicated by the dotted lines I5. If rthe base is moved upwardly the arm i4 will occupy the dotted position indicated by the reference numeral II. It will be seen, therefore, that the relative movement of the base 2t will include a longer or shorter arc of the arcuate resistor between the brushes I6 and I8 depending on the direction of movement of the base. vIf the base is moved downwardly the arcuate segment will be less than a semicircle. If the base is moved upwardly the arcuate segment embraced by the 6. 180 sweep. Ii this value oi R is less than thel desired value which is located. at point 202 on the curve 203, it will be necessary to move the base member 20 upwardly so that a sweep of 180 of the brush will include more than 180 of the arcuate resistor i0 in an amount sufficient to bring the overall. resistance to the value indicated by the point 202. Since the center or rotation of the brush is now eccentric to the locus oi the arcuate resistor, the resistance will vary as a sine curve shown by the curve 203 of Figure 3. Since the sine of is one, the resistance at point 264-, the 90 poi-nt, will be exactly correct. One of the brushes is made to coincide with point 9 during the adjustment, so that the Zero degree point will always be correct. The amount of the adjustment is ina-de so that the point will coincide with point 2M to give a correct resistance value at this point. Accordingly, the resistance will have a correct value at the zero point, the 90 point, and the 180 point. The slight variation in resistance at intermediate angles is small since the sine of the angle of displacement of the base with respect to the center of rotation of the brushes is usually quite small. The overall error will in all cases` be small and much better than that of any known variable resistor. A variable resistor having a linearity of winding within .1% and together with an overall resistance of 1%, is considered excellent. If a variable resistor of this Construction were used as a resistance element I0, the error in values betweenY zeroy and R, would be no better than that obtained by a variable resistor. Between values of R and. 2R the error is one-half, between values of 2R and 3R the errors are reduced to one-third, and. at values between 'YR and 8B. the errors are reduced to one-eighth. Il an extremely accurate variable resistor is required the units of R may be reduced. to a small fraction of the overall resistance involved and an extremely high average accuracy will be obtained by means of my construction, as can readily be appreciated. Furthermore, since only one variable resistor is required, this can' be constructed to careful tolerances and the accuracy of the entire system will be greatly improved.

It will be seen that I have accomplished the objects of my invention. I have provided a variable resistor having a desired overall resistance within narrow limits and which has greater linearity so that a predetermined movement of a variable brush will produce a corresponding change in resistance within narrow limits. I have provided a simple and expeditious means for adjusting the overall resistance `oi the variable resistor without sacricing accuracy percentagewise with respect to the whole assembly.

It Will be understood that certain features and sub-combinations are of utility and may be employed without reference to other features and sub-combinations. This is contemplated by and is within the scope of my claims. It is further obvious that various changes may be made in details-within the scope of my claims without departing from the spirit of my invention. It is, therefore, to be understood that my invention is not to 'be limited to the specific details shown and described.

Having thus described my invention, what I claimV is:

1. A variable resistor having two terminals across which resistance is varied, including in combination a rheostat having a resistance elemen-t, a brush, means `for traversing said brush successively in effective contact with said resistance element, a group of fixed resistors, means for connecting the fixed resistors successively in cumulative series, means for operating said connecting means in phase with the traversing movement of said brush, means for connecting said brush to one end of the group of said fixed resistors, means for connecting one variable resistor terminal to one end of the rheostat resistance element, a switch for successively connecting the other variable resistor terminal to the other end of the group of series Iconnected resistors, and means for operating said switch in phase with the traversing movement of said brush.

2. A variable resistor having two terminals across which resistance is varied, including in combination a rheostat having a resistance element, a pair of brushes, means for traversing said brushes successively in effective contact with said resistance element, a pair of groups of fixed resistors, means for connecting the fixed resistors of respective groups successively in cumulative series, means for operating said connecting means in phase with the traversing movement of said brushes, means for connecting one of said brushes to one end of a group of fixed resistors, means for connecting the other oi said brushes to one end of the other group of fixed resistors, means for connecting one variable resistor terminal to one end of the rheostat resistance element, a switch for alternately connecting the other variable resistor terminal to the other end of one group of series connected fixed resistors, and means for operating said switch in phase with the traversing movement of said brushes.

3. A variable resistor as in claim 2, in which said brushes are positioned 180 apart and the value of the resistance element between said brushes is R, the value of the rst fixed resistor of one group of fixed resistors is R, and the value of successive fixed resistors of that group is 2R, the value of the rst fixed resistor of the other group of iixed resistors is zero and the value of each of the other fixed resistors of the second group is 2R.

4. A variable resistor as in claim 2, in which said brushes are positioned on an insulating arm and spaced substantially 180 from each other, and the means for traversing said brushes successively in contact with the resistance element includes means for rotating said arm.

5. A variable resistor as in claim 2, in which said means for connecting said fixed resistors successively in cumulative series includes a plurality of conductors each connected to two adjacent resistors at an inter-resistor point, and a brush for successively contacting said conductors.

6. A variable resistor as in claim 2, in which said means for connecting said fixed resistors successively in cumulative series includes a plurality of conductors each connected to two adjacent resistors at an inter-resistor point, a brush for successively contacting said conductors, and said means for operating said brush comprises means movable as a function of the movement of the rheostat brushes.

7. A variable resistor as in. claim 2, in which said means for operating said switch comprises a cam and means for rotating said cam as a function of the traversing movement of said rheostat brushes.

8. A variable resistor as in claim 2, in which said brushes are positioned 180 from each other and means for varying the resistance of said rheostat resistance element which is included between said brushes.

9. A variable resistor as in claim 2, in which` the resistance element of said rheostat is disposed- 10. A variable resistor as in claim 2, in which said switch comprises a quick acting switch lwhereby to reduce the time interval during which the circuit is interrupted in the shifting of the connection of the variable resistor terminal from the end of one group of series connected xed resistors to the end of the other group of series connected xed resistors.

11. A variable resistor as in claim 2, in which said means for connecting the fixed resistors of one group successively in cumulative series is operated in iixed phase relation to the operation of the means for connecting the other group of iixed resistors successively in cumulative series.

12. In a variable resistor, a rheostat having a resistance element, a pair of movable brushes adapted successively to traverse said resistance element to vary the resistance between a brush and one end of the resistance element, a, pair of groups of a plurality of fixed resistors each resistor having a predetermined resistance value, switching means adapted alternately to connect the xed resistors of respective groups successively in cumulative series with the rheostat resistance element at predetermined points of a brush travel over the rheostat element.

13. A variable resistor having two terminals across which resistance is varied, including in combination a rheostat having a resistance element, a pair of brushes, means for traversing said brushes successively in effective contact with said resistance element, a pair of groups of iixed resistors, means for connecting the xed resistors of respective groups successively in cumulative series, means for operating said connecting means in phase with the traversing movement of said brushes, means for connecting one of said brushes to one end of a group of fixed resistors, means for connecting the other of said brushes to one end of the other group of fixed resistors, means for connecting one variable resistor terminal to one end of the rheostat resistance element, a switch for alternately connecting the other variable resistor terminal to the other end of one group of series connected fixed resistors, means for operating said switch in phase with the traversing movement of said brushes, said means for connecting the fixed resistors of respective groups in cumulative series including a pair of brushes and a plurality of segments, and means for interconnecting the zero segments to give a zero resistance with either brush.

CARL F'. SCHAEFER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,671,146 Crocker May 29, 1928 2,366,968 Kaufmann Jan. 9, 1945 2,498,967 Schaefer Feb. 28, 1950 2,501,813 Franklin Mar. 28, 1950 

